This invention relates to a master cylinder wherein a piston assembly has first and smaller second surface areas that are combined to define a first effective area while the smaller second surface area defines a second effective area in a first chamber and where operational pressurized hydraulic fluid presented to the first chamber is directly communicated to wheel brakes in a first brake circuit and acts on the first effective area to pressurize fluid in a second chamber that is communicated to wheel brakes in a second brake circuit to effect a first brake application in response to movement of an input member by a force applied to a brake pedal and where movement of the input member by a manual force, in an absence of the presence of operational pressurized hydraulic fluid in the first chamber, moves the second effective area into the second chamber to pressurize fluid therein that is communicated to the wheel brakes in the second brake circuit.
The present invention is of a type hydraulic brake boosters referred to as a xe2x80x9cfull-powerxe2x80x9d brake booster, of which the following U.S. Patents are considered to be typical: U.S. Pat. Nos. 4,441,319; 4,490,977; 4,514,981; 4,665,701; 4,685,297; 4,704,867; 4,724,674; 5,526,731 and 5,927,074. In such brake boosters, an accumulator is charged with fluid pressure and selective activated through movement of a control valve by an input member to present pressurized fluid to an actuation chamber that acts on an actuation piston. The actuation piston in turn provides a force for moving piston in a master cylinder that pressurizes operational fluid that is presented to wheel brakes to effect a brake application. The operational pressure that is developed is proportional to the force applied to the actuation piston and inversely proportional to the cross-sectional area of the piston in the master cylinder for a given force applied to an input member by an operator to move the control valve. The resulting travel of the input member and brake pedal is proportional to the travel of the actuation piston in the master cylinder. Conversely, the travel of the piston in the master cylinder is also proportional to the displacement of the fluid in the brake system at any given pressure and inversely proportional to the cross-sectional area of the actuation piston.
Thus, the travel of the input member and brake pedal is inversely proportional to the cross-sectional area of the actuation piston. Given these facts, in order to achieve less travel of the input member it is necessary to have a larger cross-sectional area for the pistons in the master cylinder. However with an increase in the size of the pistons in the master cylinder to produce a larger or increased output force, a same increase is necessary for an input force for effecting a brake application. An increase in the size of the pistons would not significantly effect the development of a brake application when pressurized fluid stored in an accumulator is available to provide a boost force, however, during a manual mode when the force is provided by an operator, under some circumstances an operator may not be capable of or have sufficient strength to develop an input force to produce an output force to meet braking requirement set by FMVSS-105 and FMVSS-135.
The present invention overcomes a disadvantage in the prior brake systems when switching from a power assist mode to a manual mode to effect a brake application by providing a master cylinder with a piston assembly having a first cross-sectional area that is utilized during a power assist mode of operation to effect a first brake application and a smaller second cross-sectional area that is utilized during a manual mode of operation to effect a second brake application.
According to this invention, the master cylinder in a first embodiment has a housing with a bore therein for retaining a first piston assembly to define a first chamber and a second chamber. The first chamber is connected through a first outlet port to wheel brakes in a first brake circuit while the second chamber is connected through a second outlet port to wheel brakes in a second brake circuit. The first piston assembly includes a first sleeve that has a concentric first cylindrical body located therein. The first sleeve has a first surface area and the first cylindrical body has a second surface area that are combined to define a first effective area within the first chamber such that the diameter of the first sleeve is substantially equal to the diameter of the bore. The second surface area of the first cylindrical body has a diameter that is smaller than the sleeve and defines a second effective area within the first chamber. In the master cylinder in a second embodiment, a second piston assembly that engages the first piston assembly and includes a second sleeve with a concentric second cylindrical body located therein. The second sleeve has a diameter equal to the first sleeve and as a result the surface area of the second sleeve and second cylindrical body which is located in the second chamber is equal to the first effective area. The surface area of the second cylindrical body that is located in the second chamber may or may not be equal to the surface area of the first cylindrical body located in the first chamber but in a preferred use such are substantially equal. In both the first and second piston assembles, the first cylindrical body has a blind bore therein that carries a valve to control communication of pressurized fluid from a source of operational hydraulic fluid to the first chamber. During a first or power assist mode of operation, the valve is activated by movement of an input member connected to a brake pedal to corresponding allow operational hydraulic fluid to be directly communicated to the wheel brakes in the first brake circuit by way of the first chamber to initiate a first brake application. The operational hydraulic fluid presented to the first chamber also simultaneously acts on first effective area of each piston assembly and moves such piston assembly toward the second chamber. After closing a compensation port to the second chamber, the first effective area of the piston assembly in the second chamber pressurize fluid therein and develops a first pressurized fluid that is communicated to wheel brakes in the second circuit to assist in effecting the first brake application. In a second or manual mode of operation the master cylinders in the first and second, embodiments function to effect a brake application when operational hydraulic fluid is not available for presentation to the first chamber. In a manual mode, an input force applied to the brake pedal by an operator is transmitted by the input member into the first cylindrical body and directly moves the second effective area into the second chamber to pressurize fluid therein and after closing of the compensation post develop pressurized fluid that is communicated to the wheel brakes in the second brake circuit to effect a second brake application. The ratio of the first effective area to the second effective area is selected such the pedal travel necessary to achieve a pressure level of the fluid in the second chamber in the manual mode is about twice the travel as required during a power assist mode. In the second embodiment of the piston assembly, a space is located in the abutment of the first sleeve and concentric first cylindrical body and the second sleeve and second cylindrical body to define a secondary actuation chamber. The secondary actuation chamber is connected to the source of operational hydraulic fluid through an electronic control valve under the control of an electronic control unit (ECU) for the vehicle. The ECU receives various inputs relating to conditions that may effect a safe operation of the vehicle. The inputs are analyzed by the ECU and if conditions dictate a signal to effect a brake application is developed to attenuate or at least reduce the effect of the conditions on the operation of the vehicle. The electronic control valve on being activated by the ECU allows pressurized operational hydraulic to be communicated to the secondary actuation chamber. The fluid pressure of the operational hydraulic fluid simultaneously act on the first effective area first effective area defined by the second sleeve and second cylindrical body and the first cylindrical body to move the second sleeve and second cylindrical body toward the second chamber and the first cylindrical body toward the first chamber. Movement of the first sleeve and its first cylindrical body toward the first chamber opens the first valve carried by the first cylindrical body to allow operational hydraulic fluid to be communicated from the inlet port directly to the wheel brakes in the first circuit to effect a brake application per the ECU. Similarly, movement of the second sleeve and second cylindrical body toward the second chamber pressurizes fluid in the second chamber to develop pressurized fluid that is supplied to the wheel brakes in the second circuit to effect a brake application per the ECU. Once the situation has abated or been reduced as indicated by the signals supplied to the ECU, the ECU terminates the activation of the electronic control valve and the actuation chamber is communicated to the reservoir such that the return spring brings second sleeve and second cylindrical body into engagement with the first sleeve and first cylindrical body to reestablish a position of rest for the piston arrangement.
An advantage of this invention resides in a master cylinder having a single piston assembly wherein a plurality of wheel brakes in a first brake circuit are directly supplied with operational hydraulic fluid and wheel brakes in a second brake circuits are supplied with pressurized fluid developed by the operational hydraulic fluid to effect a brake application.
An object of this invention is to provide a master cylinder having a piston assembly through which first and second brake circuits are provided with pressurized fluid in response to either an operator input or an electronic input to effect a brake application wherein a substantially identical pressurized fluid level is developed to effect a brake application in at least the first brake circuit is substantially identical.
A further object of the invention is to provide a master cylinder with a piston assembly whereby the distance of the travel of the input member during a hydraulic boost mode to develop a first pressurized fluid is about one half of the distance of the travel of the input member during a manual mode to develop a second pressurized fluid having a substantially same pressure level.